1. Field of the Invention
This invention relates generally to gas separation by electrostatic precipitators having electrode retaining supporting means and more particularly to electrostatic precipitators that remove corrosive particles from a corrosive particle laden gas passing therethrough having improved discharge and collector electrode system and gas distribution means.
2. Description of the Prior Art
Industry generally utilizes two types of electrostatic precipitators for purifying industrial gases prior to the gases being released to the atmosphere. One such type of electrostatic precipitator utilizes the plurality of spaced collector electrode plates suspended within the precipitator with a plurality of discharge electrode wires suspended between adjacent plates. This type of precipitator is used to remove particles from a gas that is essentially non-acidic. The second type of precipitator utilizes a plurality of acid resistant collector electrode tubes suspended within the precipitator. Each tube has a single acid resistant discharge wire suspended through its center. This invention is limited to improvements designed to enhance the efficiency of the second type precipitator. When a corrosive particle laden gas is passed through the collector electrode tube of this second type precipitator, a voltage is applied to the discharge wires running through the center of each of the collector electrode tubes, causing an electrostatic field to be created within the collector electrode tubes. When the electrostatic field has been created, it ionizes the acid particles within the gas and these charged particles are collected on the inside surface of the collector electrode tubes. The particles are disposed of by letting the wet acid particles run down the inside surface of the collector electrode tube to be collected at the bottom of the precipitator for disposal.
It is a common phenomenon that when the discharge wires are electrofied, they tend to oscillate or swing due to the electrical field created. This oscillation causes a non-uniform electrical field and may cause arcing between the discharge electrode wire and the collector electrode tube when the discharge wires come in close proximity to the collector electrode tube. This arcing is detrimental to the collector electrode tube in that it produces holes therein and results in inefficient precipitator operation.
In the tube type precipitator it is common practice to suspend a rigid tubular support grid from the top of the precipitator just above the top of the collector electrode tube. The single discharge wires are suspended from the top support grid through the center of the collector electrode tubes. A weight is suspended from the bottom of each discharge wire to maintain the discharge wire straight. The stiffness of the discharge wire and the combined mass of the discharged wire and the attached weight offset the oscillating tendency described above. In addition, the lower end of the discharge wires are guided by a rigid tubular grid located below the collector electrode tubes. The guide grid is suspended from the top support grid by a plurality of rigid tubular support pipes. The grid system is covered by an anti-corrosive material such as lead to prevent the acid from damaging the grid system. In addition, the interior of the precipitator shell is lined with a lead covering to prevent acid from contacting the non-corrosive resistant metal shell. Further, the collector electrode tubes and discharge wires are commonly made of lead.
It is common practice to place each of the lead covered support pipes through the center of the corresponding number of collector electrode tubes to eliminate the need to insulate the supports from the shell since the support pipes are carrying high voltage electrical current and to utilize the space occupied thereby for collecting purposes thus increasing the overall efficiency of the precipitator. Standard 10 inch collector electrode tubes are used with a single discharge wire through the center and standard 13 inch diameter collector electrode tubes are used with the support pipes extending through the center thereof.
In the past, several types of support members have been used to try and enhance the electrical emission by each of the support members. First, the support pipe was fabricated from two steel angle beams welded at the apices and covered with a lead sheet to protect the angle beams from the corrosive elements. Four conventional lead star wires which are used as the standard discharge collector electrode wire were burned to the exterior surface of the lead sheet. It was found that the electrical emission efficiency from the four star wires was not satisfactory. A second type of support pipe was fabricated from a steel pipe which was covered with a lead sheet. Again, four star wires were burned to the lead sheet at substantially 90.degree. angles. The results were better than that previously used but were not satisfactory in that they did not increase the emission efficiency of the support pipe. The third type support pipe was again a steel pipe covered with a lead sheet to protect it from the corrosive elements of the gas passing around it. However, eight star wires were burned to the sheet lead cover at 45.degree. angles. The result improved the overall electrical emission efficiency of the support pipe. However, they were not entirely satisfactory and did not produce the same precipitating efficiency as the single discharge star wire in the 10 inch diameter collector electrode tube. In addition, the third type discharge support pipe creates a number of other disadvantages. It is quite time consuming and expensive to burn the desired number of discharge wires to the lead covering of the support pipe. There are usually four support pipes supporting the bottom grid guide and since there are eight discharge wires for each support pipe, it makes a total of 32 discharge wires that must be attached. Another disadvantage is that it has not been possible to substantially simulate the gap between the discharge electrode wire and the inside surface of the 10 inch diameter collector electrode tube in the 13 inch diameter collector electrode tube when using the standard pipe and lead sheet covering. The radial distance between the discharge wires burned to the lead covering of the support pipe and the inner surface of the 13 inch collector electrode tube is less than the radial distance between the single discharge wire and the inner surface of a 10 inch collector electrode tube. Thus, a lower voltage is generated in the 13 inch collector electrode tube than in the 10 inch collector electrode tube and the voltage supplied to the support pipes will control the voltage in all the collector electrode tubes causing inefficient precipitation in the 10 inch collector electrode tubes.
Another disadvantage is that although there are numerous configurations for discharge wires the most efficient is the shape of a five pointed star, commonly called star wire. The electrical emission producing the electrostatic field from each of the points of the star is exceptionally good. But when the star wires are burned to the lead covering on the support pipe, some of the points of the stars are pointed toward the adjacent star. The electrical emissions from these points conflict causing inefficient corona and precipitation.
Another problem associated with tube type precipitators which are used for the removal of acidic particles from an acid particle laden gas is maintaining an even distribution of gas flowing through all the collector electrode tubes within the interior of the precipitator so no one collector electrode tube handles more gas than another. Conventionally, these precipitators not only include a top support plate from which the collector tubes are suspended but also include a bottom plate secured around the bottom of each collector tube and secured to the wall of the precipitator. The gas enters through the side of the precipitator below the bottom plate so that it may enter the bottom of the collector electrode tubes. However, ordinarily there is but one inlet port for directing the gas into the precipitator below the bottom plate. Since in a conventional tube type precipitator the shell is cylindrical there will be some collector electrode tubes which are in close proximity to the inlet port and there will be a majority of collector electrode tubes which are spaced farther and farther away from the inlet port. Thus, when the gas flows through the inlet port the majority of the gas rises through those collector electrode tubes which are in close proximity to the inlet port and relatively small amounts of gas enter those collector electrode tubes that are at the opposite point from the inlet port. Thus, those collector electrode tubes that are close to the inlet port will handle a vast amount of gas flowing therethrough and the precipitation of the gas will not be complete causing some gas to escape through the top of the collector electrode tubes that still contain acid particles. In addition, since relatively little gas enters those collector electrode tubes at the far point from the inlet port, they will not be working at their maximum capacity. The lack of equal distribution of gas through all the collector electrode tubes causes inefficient precipitation.
Another disadvantage associated with collector electrode precipitators that remove acid particles from an acid laden gas is that the inside surface of the shell must be lined with some type of corrosive resistant material such as lead so that the corrosive gases will not attack the non-corrosive resistant metal shell. In addition, conventionally, the interior of the precipitator above the top support plate and below the bottom plate is at a lower or negative pressure as compared to the outside atmospheric pressure. Should a pin hole develop through the shell of the precipitator, a positive pressure will be maintained between the shell and the lead lining in the interior of the precipitator. This positive pressure acting on one side of the lead lining above the top support plate and below the bottom plate will cause the lining to pull away from the metallic shell and will eventually cause the lead lining to rupture. When this happens, the precipitator must be shut down and the damaged lining repaired and the pin hole or break in the shell must be found and sealed up.
Another problem inherent in tube type precipitators that use bottom plates is that the area above the top support plate and below the bottom plate is maintained at a negative pressure compared to the outside atmospheric pressure as stated previously while the pressure between the top and bottom plates is maintained at atmospheric pressure. This results in a negative pressure within the collector electrode tubes and a positive pressure on the outside of the collector electrode tubes. The pressure differential between the outside and inside of the collector electrode tubes causes the lead collector electrode tubes to collapse inward which necessitates shutting down the precipitator to replace the collapsed collector electrode tube.